The M3 Muscarinic Acetylcholine Receptor Can Signal Through Multiple G Protein Families

The M3 muscarinic acetylcholine receptor (M 3 R) is a G protein- coupled receptor (GPCR) that regulates important physiologic processes, including vascular tone, bronchoconstriction, and insulin secretion. It is expressed on a wide variety of cell types, including pancreatic beta, smooth muscle, neuronal, and immune cells. Agonist binding to the M 3 R is thought to initiate intracellular signaling events primarily through the heterotrimeric G protein Gq. However, reports differ on the ability of M 3 R to couple to other G proteins beyond Gq. Using members from the four primary G protein families (Gq, Gi, Gs, and G13) in radioligand binding, GTP turnover experiments, and cellular signaling assays, including live cell G protein dissociation and second messenger assessment of cAMP and inositol trisphosphate, we show that other G protein families, particularly Gi and Gs, can also interact with the human M 3 R. We further show that these interactions are productive as assessed by amplification of classic second messenger signaling events. Our findings demonstrate that the M 3 R is more promiscuous with respect to G protein interactions than previously appreciated.